Make int.gcd accept zero operands.

runtime/lib/bigint.dart

Index: runtime/lib/bigint.dart
diff --git a/runtime/lib/bigint.dart b/runtime/lib/bigint.dart
index 2e659bc76f9b1e8a7962f1db8fe358a39015deb3..3f23ad775596df347ad948095df894d679c1ce1c 100644
--- a/runtime/lib/bigint.dart
+++ b/runtime/lib/bigint.dart
@@ -1558,11 +1558,9 @@ class _Bigint extends _IntegerImplementation implements int {
         throw new UnsupportedError("Not coprime");
       }
     } else {
-      if (x_used == 0) {
-        throw new ArgumentError.value(0, "first operand", "must not be zero");
-      }
-      if (y_used == 0) {
-        throw new ArgumentError.value(0, "second operand", "must not be zero");
+      if ((x_used == 0) && (y_used == 0)) {
+        throw new ArgumentError.value(0, null,
+                                      "at least one operand must not be zero");

[floitsch, 2015/06/24 12:18:13]

Wolfram Alpha goes as far as to say that gcd(0, 0) => 0

[Lasse Reichstein Nielsen, 2015/06/24 12:44:10]

It's a usable definition, as long as you can explain to the user that 0 is the
greatest divisor of all (by being the top element in the lattice of integers
ordered by divisibility, and gcd being the meet operation on that lattice).
Well, or we can just say it's a special case :)

On the other hand, it does avoid throwing a not very helpful error message.

       }
       if (((x_used == 1) && (x_digits[0] == 1)) ||
           ((y_used == 1) && (y_digits[0] == 1))) return 1;
@@ -1810,6 +1808,9 @@ class _Bigint extends _IntegerImplementation implements int {
     if (other is! int) {
       throw new ArgumentError.value(other, "other", "not an integer");
     }
+    if (other == 0) {

[floitsch, 2015/06/24 12:18:13]

Are we sure that bigints cannot be 0?
(If yes assert it, and add comment).

[Lasse Reichstein Nielsen, 2015/06/24 12:44:10]

They can be 0, at least internally. The `other._toBigInt()` call below would be
used for 0 before this change. 
I don't think an externally visible BigInt will be 0. The call to _toValidInt()
above is there to prevent that.

[regis, 2015/06/24 16:07:16]

Correct. At this point, 'other' has not yet been forced to Bigint (either here
or in the forwarding gcd of integers.dart), and 0 is still Smi. We would assert
fault when using '== 0' otherwise.

+      return this.abs();
+    }
     return _binaryGcd(this, other._toBigint(), false);
   }
 }